Effetti cardiovascolari delle malattie tiroidee nel cane e nel gatto

Abstract

Cardiovascular system owns the receptors for thyroid hormones, and it is sensitive to changes in their blood concentration. Thyroid hormones affect myocardial contractility, diastolic function, systemic vascular resistance and heart rate through genomic and non-genomic mechanisms. Genomic mechanisms on cardiac myocytes include the expression of genes encoding contractile proteins, ion transporters and  adrenergic receptors, exerting positive inotropic, lusitropic and chronotropic effects. Non-genomic actions comprise stimulation of specific ion channels, enhancing myocardial function; in addition, they can cause relaxation of vascular smooth cells, leading to decreased systemic vascular resistance. Consequently, both deficiency and excess of thyroid hormone can result in changes in cardiac function and cardiovascular hemodynamics. Hyperthyroidism, which is common in cats, is characterized by a high cardiac output state with the increase in contractility, preload and heart rate, and the reduction in systemic vascular resistance, resulting in a hyperdynamic circulation. This can lead to myocardial remodeling with left ventricular hypertrophy, diastolic dysfunction, arrhythmias, hypertension and the risk of thrombosis. On the other hand, hypothyroidism, which is recognized as a frequent endocrine disorder in dogs, is associated with reversible decrease in myocardial contractility, peripheral vasoconstriction and increased arterial stiffness, that is an important determinant of atherosclerosis. These physiopathological effects induced by hyperthyroidism and hypothyroidism are responsible for important clinical cardiovascular findings on physical examination; in addition, electrocardiographic, echocardiographic and radiographic abnormalities are reported in affected patients. Most of the cardiovascular changes are reversible when the endocrinopathy is controlled with antithyroid agents or replacement therapy; however, more severely affected patients may require specific treatment for congestive heart failure, that is the final clinical event of the greatest part of cardiac diseases, and it can be also associated with thyroid disorders.